1. Field of the Invention
The present invention relates to stabilization and/or translation zones for contact lenses requiring rotational stability and linear movement on the eye, such as toric contact lenses, and more particularly to contact lenses requiring rotational stability and linear translation and incorporating one or more dynamic stabilization and/or translation zones that have varying physical properties.
2. Discussion of the Related Art
Myopia or nearsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point before they reach the retina. Myopia generally occurs because the eyeball or globe is too long or the dome of the cornea is too steep. A minus powered spherical lens may be utilized to correct myopia. Hyperopia or farsightedness is an optical or refractive defect of the eye wherein rays of light from an image focus to a point after they reach or behind the retina. Hyperopia generally occurs because the eyeball or globe is too short or the dome of the cornea is too flat. A plus powered spherical lens may be utilized to correct hyperopia. Astigmatism is an optical or refractive defect in which an individual's vision is blurred due to the inability of the eye to focus a point object into a focused image on the retina. Unlike myopia and/or hyperopia, astigmatism has nothing do to with globe size or cornea steepness, but rather it is caused by an abnormal curvature of the cornea. A perfect cornea is spherical whereas in an individual with astigmatism, the cornea is not spherical. In other words, the cornea is actually more curved or steeper in one direction than another, thereby causing an image to be stretched out rather than focused to a point. A cylindrical lens rather than a spherical lens may be utilized to resolve astigmatism.
A toric lens is an optical element having two different powers in two orientations that are perpendicular to one another. Essentially, a toric lens has one power, spherical, for correcting myopia or hyperopia and one power, cylinder, for correcting astigmatism built into a single lens. These powers are created with curvatures at different angles which are preferably maintained relative to the eye. Toric lenses may be utilized in eyeglasses, intraocular lenses and contact lenses. The toric lenses used in eyeglasses and intraocular lenses are held fixed relative to the eye thereby always providing optimal vision correction. However, toric contact lenses may tend to rotate on the eye thereby temporarily providing sub-optimal vision correction. Accordingly, toric contact lenses also include a mechanism to keep the contact lens relatively stable on the eye when the wearer blinks or looks around.
It is known that correction of certain optical defects may be accomplished by imparting non-rotationally symmetric corrective characteristics to one or more surfaces of a contact lens such as cylindrical, bifocal, multifocal, wavefront corrective characteristics or decentration of the optical zone. It is also known that certain cosmetic features such as print patterns, markings, and the like are required to be placed in a specific orientation relative to the wearer's eye. The use of contact lenses is problematic in that each contact lens of the pair must be maintained at a specific orientation while on the eye to be effective. When the contact lens is first placed on-eye, it must automatically position, or auto-position, itself and then maintain that position over time. However, once the contact lens is positioned, it tends to rotate on the eye due to the force exerted on the contact lens by the eyelids during blinking as well as eyelid and tear film movement.
Maintenance of the on-eye orientation of a contact lens typically is accomplished by altering the mechanical characteristics of the contact lens. For example, prism stabilization, including decentering of the contact lens' front surface relative to the back surface, thickening of the inferior contact lens periphery, forming depressions or elevations on the contact lens' surface, and truncating the contact lens edge, are all methods that have been utilized.
Additionally, static stabilization has been used in which the contact lens is stabilized by the use of thick and thin zones, or areas in which the thickness of the contact lens' periphery is increased or reduced, as the case may be. Typically, the thick and thin zones are located in the contact lens' periphery with symmetry about the vertical and/or horizontal axes. For example, each of two thick zones may be positioned on either side of the optic zone and centered along the 0-180 degree axis of the contact lens. In another example, a single thick zone positioned at the bottom of the contact lens providing a similar weight effect, like that of prism stabilization, but also incorporating a region of increasing thickness from top to bottom in order to utilize upper eyelid forces to stabilize the contact lens may be designed.
The challenge with static stabilization zones is a tradeoff between contact lens stability and comfort, plus the physical limitations associated with increased thickness. With a static stabilization zone, the slope of the stabilization zone is fixed in the contact lens. Changes to the design to improve rotational speed, such as increasing the surface slope of the stabilization zone, also increases contact lens thickness and may adversely impact comfort. Additionally, the contact lens design has to accomplish two things; namely, to rotate to the proper orientation on insertion, and to maintain that orientation through the wear period. A static design requires tradeoffs in performance between these two modes.
Contact lenses may also be worn to address presbyopia. In one type of such lenses, distance and near vision regions are concentrically arranged around the geometric center of the lens. Light passing though the optical zone of the lens is concentrated and focused at more than one point in the eye. These lenses are generally used in simultaneous vision mode. In simultaneous vision, portions of the lens optical zone focused for distance and near are available at the same time, focusing light from both object distances simultaneously. This is disadvantageous as image quality and image contrast may be degraded.
In another type of contact lens meant to address presbyopia, the lens alternates power between distance and near, or distance, near and intermediate. One type of alternating vision lens comprises an optic portion which changes power due to an external stimulus.
In another type of alternating power contact lens; namely, a segmented lens, near and distance vision regions are not concentric about the geometric center of the lens. The wearer of the segmented lenses is able to access the near vision region of the lens because the lens is constructed to allow it to translate, or move vertically relative to the pupil of the wearer's eye. This translating lens moves vertically when the person wearing the lens shifts their gaze downwardly, for example, to read. This upwardly positions the near vision portion in the center of the wearer's gaze. Substantially all of the light passing though the optical zone may be focused at a single point in the eye based on gaze.
One type of translating lens has a truncated shape. That is, unlike most lenses that are substantially continuously circular or oval, the lower portion of the truncated contact lens if flattened by cutting off or shortening that part of the lens. This results in a substantially flat and thicker edge at the bottom of the lens. It is this thicker edge that interacts with the lower eyelid to achieve the required translation. Exemplary descriptions of such lenses are set forth in a number of patents, including U.S. Pat. No. 7,543,935, U.S. Pat. No. 7,434,930, U.S. Pat. No. 7,052,132, and U.S. Pat. No. 4,549,794. However, a relatively flat and thicker edge on contact lenses such as these may tend to reduce comfort.
Accordingly, it would be advantageous to design a contact lens with dynamic stabilization zones that auto-position the contact lens quickly and hold and/or maintain the desired position for optimal visual acuity regardless of eye movement, blinking and tears. In addition, it would also be advantageous to design a contact lens with a compliant dynamic translation zone for ensuring proper linear movement. It would also be advantageous to design the both dynamic stabilization and translation zones for comfortable interaction with the eyelids.